By MATTHEW L. WALD

Published: April 28, 2005

A material being used at 4 nuclear reactors in New York and 10 others around the country to prevent fire damage to vital equipment would shrink during a fire and expose the equipment to unacceptable amounts of heat, according to the Nuclear Regulatory Commission.

The discovery that the material, sold under the trade name Hemyc, does not provide enough protection to meet the commission's rules, follows a major effort to replace a different material, Thermo-lag, used for the same purpose. Problems with Thermo-lag were discovered in 1992.

''If we were smarter we would have not allowed this stuff to be used in the first place,'' Brian W. Sheron, associate director of the project licensing and technical analysis office of the commission, said in an interview on Tuesday. As potential problems are discovered, he said, the commission moves to make the reactor owners correct them, at a pace that depends on estimates of the degree of the hazard. In this case, the immediate risk may be small, commission officials said.

The commission was prompted to study Hemyc, Mr. Sheron said, when an inspector noticed that it did not look much different from other material being replaced. Hemyc is made of a silicon and a ceramic. The commission plans a meeting Friday with reactor owners to ask them to demonstrate why their plants should not be shut down until the problem can be fixed.

Hemyc, which is produced in blanket form, is often wrapped around cable trays or junction boxes and stitched together. Commission rules require that cables that power or control crucial equipment, usually pumps or valves, be protected with a fire barrier that would last an hour if the area has fire-detection equipment and sprinklers or other suppression systems. In other areas, the fire protection material is supposed to last three hours.

According to tests commissioned by the agency, Hemyc shrinks by about 8 percent if exposed to temperatures of about 800 degrees Fahrenheit and could then expose the cables. It is not clear whether a fire could reach that temperature, but the material does not meet the agency's standard. Mr. Sheron said that Hemyc was available in a form that was already shrunk and thus would not shrink further when heated, but that a different form was found in the reactors.

In New York, Hemyc is used at Indian Point 2 and 3, in Buchanan, and the James A. FitzPatrick reactor, near upstate Oswego. All three of those plants are operated by Entergy, and a spokesman for the company, James Steets, said Entergy had established fire patrols in the area where Hemyc was used until it can determine the extent of the hazard and decide whether replacement is warranted.

The Robert E. Ginna plant near Rochester also uses the material.

Fires are believed to be one of the most important threats to nuclear reactors. In March 1975, a fire in a cable room under the control rooms for two of the Tennessee Valley Authority's reactors at Browns Ferry in Alabama burned for more than six hours and destroyed cables that controlled the operation of crucial pumps and valves, or supplied them with electric power.

David Lochbaum, a nuclear engineer at the Union of Concerned Scientists, a nonprofit group that is often critical of nuclear safety, said it was not clear why the commission was still discovering inadequate materials 30 years after the Browns Ferry fire. ''It would sure be nice someday to get a one-hour fire wrap that works,'' he said.

Photo: Indian Point 2 and 3 are among the 14 reactors in the country that use the material. (Photo by Suzy Allman for The New York Times)